selftests/bpf: Test_progs option for listing test names

The program test_progs have some very useful ability to specify a list of
test name substrings for selecting which tests to run.

This patch add the ability to list the selected test names without running
them. This is practical for seeing which tests gets selected with given
select arguments (which can also contain a exclude list via --name-blacklist).

This output can also be used by shell-scripts in a for-loop:

 for N in $(./test_progs --list -t xdp); do \
   ./test_progs -t $N 2>&1 > result_test_${N}.log & \
 done ; wait

This features can also be used for looking up a test number and returning
a testname. If the selection was empty then a shell EXIT_FAILURE is
returned.  This is useful for scripting. e.g. like this:

 n=1;
 while [ $(./test_progs --list -n $n) ] ; do \
   ./test_progs -n $n ; n=$(( n+1 )); \
 done

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159363985751.930467.9610992940793316982.stgit@firesoul